Before buying, think about your typical training day. Ask:

• How many sessions do I run, and how long are they?
• Do I need a device that fits in a shirt pocket?
• Am I okay with a smaller battery if it means lighter carry?

If possible, handle a compact device in person. Feel its weight and size. Even better, do a test session with one to see how it performs.

If you're buying online, read reviews from other trainers or users with similar needs.